Dental impression composition



Patented Mar. 11, 1941 UNITED STATES PATENT OFFICE DENTAL IMPRESSIONCOMPOSITION California No Drawing Application October 28, 1939, SerialNo. 301,836

9 Claims.

This invention relates to reversible dental impression materials andpertains more particularly to dental impression compositions of thehydrocolloid type. This application is a continuationin-part of myallowed application Serial No. 207,311, filed May 11, 1938, allowedAugust 24, 1939, for Dental impression composition.

The principal object of the present invention is to provide ahydrocolloid dental impression composition of high breaking strength andresistance to plastic deformation, and of advantageous characteristicswith respect to the setting of gypsum plaster brought in contacttherewith in the reproduction of models.

The hydrocolloid dental impression compositions, of which thecompositions of the present invention are types, are characterized bybeing resilient solid gels at temperatures below normal human bodytemperature, and capable of conversion to a sol condition attemperatures adjacent the boiling point of water. The material isreversible in nature, and will gel when cooled to just below normalhuman body temperature and may be reconverted into a fluid sol and backagain into the gel condition by repeated heating and cooling. Theprincipal constituent employed in such impression compositions is areversible gel-forming hydrocolloid, such as agaragar, Irish moss, orIceland moss, which, when made up in water, will furnish a gel which maybe used to faithfully reproduce objects in dimension, but such plain orsimple gels'have little strength and will fracture under the deformationcustomarily encountered in dental practice. For this reason it isnecessary to add some filler material to the agar-agar gel such as waxesor the like as are disclosed in U. S. Patent to Poller No. 1,672,776, ora reversible thermoplastic material such as is disclosed in U. S. patentto Harrison No. 2,021,059.

The principal efiect of these filler materials is that of improving ormodifying the solidifying characteristics (development of body orconsistency) of the material as it is cooled to the solidifyingtemperature of the hydrocolloid base material, as more particularlydescribed in the above-mentioned Harrison patent. In general, thestrength increase directly due to the addition of such fillers is rathersmall, wherefore 50 while the so-modified composition is preferablefiller materials as above described. A further object of the inventionis to rovide a modified hydrocolloid gel in which the desired strengthand toughness may be produced without the aid of filler materials, andwhich is further characterized by an improved development of body as thegel passes through the solidification range and enters the gel phase.

A further object of the invention is to provide a dental impressionmaterial of the character set forth, having improved characteristicswith respect to the setting of gypsum plaster in contact with theimpression surfaces wherefore improved plaster casts may be obtained,having a smooth, hard surface which is advantageous in the reproductionof the detail and dimensions of the subject.

There is a group of materials which are known to greatly increase thestrength and toughness of hydrocolloid gels formed with vegetablemucilages, which mucilages usually have polymers of derivatives of thehexose sugars as their .main constituents, such as agar-agar, Irishmoss, or Iceland moss gels. Certain alkaline silicates have been foundto toughen these gels,.as have hydroxides of the alkali metals, butthese particular agents are disadvantageous in a dental impressioncomposition inasmuch as the resulting material is in general toostrongly alkaline to be used in a patients mouth without discomfort orburning if sufficient of the agent is added to obtain a significanttoughening effect. Borax (sodium tetrabor'ate 'decahydrate) has beenfound effective in securing this desired increase in strength andtoughness of such hydrocolloid gels, but the presence of boraxheretofore has been found to prevent the proper setting of gypsumplasters, and at the same time a boraxmodified gel is caused to have alesser resistance to plastic deformation whereby the impressionsproduced with such compositions undergo distortion and permanentdeformation when the solidified or gelled impression is pulled from thesubject (for example, the teeth), wherefore the accuracy of reproductionis substantially lost.

According to the present invention I incorporate in such a hydrocolloidgel a borate-containing compound, either ordinary borax or any otherborate-containing compound, preferably together with a plaster-hardeningagent, whereby both the desired strength characteristics and behaviortowards dental plaster are obtained. Alkali-meta1 sulphates, such aspotassium sulphate have heretofore been employed in conventionalhydrocolloids for the purpose of improvingthe behavior of such gels withrespect to dental plaster, as exemplified by U. S. Patent No. 2,089,552,issued to Laurence E. Harrison, but I have found that when the pH of theboratecontaining hydrocolloid material is established within certainbroad limits the presence of a plaster-hardening agent, particularly analkalimetal sulphate, is found not only to cause the gel to have asatisfactory behavior with respect I v to plaster-setting, but to causethe gel to have high strength characteristics, both as to ultimaterupture and as to plastic deformation.

The effect of the alkali-metal-su'lphate is believed to be partlyphysical and partly chemical. It is known that high pH is detrimental tothe proper setting of gypsum plaster, and when an alkali-metal sulphateis incorporated in the borate-containing gels a noticeable decrease inpH is secured, which is of direct benefit as regards behavior of the gelcomposition towards plaster. Many plaster-set accelerators or hardenersare known however, and substantially all inorganic salts which increasethe solubility of gypsum in water will contribute to the improvedbehavior towards gypsum plaster of the borate-containing gels of thisinvention. Such accelerating or hardening agents as also serve todecrease the pH of the gel composition (such as the alkali-metalsulphates, and potassium sulphate in particular) are of particularbenefit. Satisfactory behavior towards plaster is conventionally definedin the dental trade as one which will cause or permit a standard (dentalgrade) gypsum plaster to set hard against the mold within about one-halfhour, and produce a casting substantially free of a powdery surface.

The broad pH range of the gel containing both a plaster-set acceleratorand a borate compound has been found to be in the neighborhood of pH 6.8to pH 8.5, although acceptable plaster behavior may be obtained withborate-containing gels in a restricted pH range of from pH 7.9 to pH 8.2in the absence of such plaster-set accelerator.

The pH of the borate-containing dental hydrocolloid may be establishedWithin the desired range :by adding to the gel composition a. boratecompound which will of itself impart the desired pI-I, or by adding botha. borate compound and a buffering agent which will establish the pHvalue of the gel material within such broad limits. I have found thatwhen the pH value of thegel is controlled, in the presence of a borateradical, the desired toughening, strengthening, and proper behaviortoward gypsum plaster is found within the limits of pH 6.8 to pH 8.6when from 1 to 4% (for example) of an alkali-metal sulphate is alsoincorporated in the gel, such alkali-metal sulphate being added to thecomposition as such, or formed therein as a reaction product, dependentupon the procedure followed in compounding the composition. Theeffective upper limit of the quantity of alkali-metal sul phate whichmay be added is apparently limited mainly by the unpleasant tasteimparted to the composition by excessive quantities, while from thestandpoint of operativeness of the material the upper limit is that ofcomplete saturation of the gel at ordinary temperatures. An alkalimetalsulphate concentration in the neighborhood of about 2% to 4% is ingeneral quite satisiactory, although when other conventional plasteraccelerators or hardeners such as common salt, alum, or the like, areemployed, greater quantities are required for equivalent performance.The effective upper limit of the concentration of borate-containingcompound in the gel has been found to be in the'neighborhood of 1%although where borate-containing compounds of relatively low borateradical contents are employed the above limit may be exceeded somewhatwithout adversely aifecting the plaster behavior of the gel.

For all practical purposes, I prefer to employ an'alkali-metal sulphatein the proportion of at a least 1% based upon the total weight of thegel composition, if the impression material is to be used in thepreparation of full or partial dentures and the like. Where thecomposition is to be employed in other uses, wherein but nominalstrength increase in the gel state is required (as compared to plainagar-agar gels) the proportion of both borate and alkali-metal sulphatemay be correspondingly reduced. For example, the incorporation of aslittle as .025% borax in an agar-agar gel showed a strength increase ofroughly 25% over a corresponding plain agaragar gel, with a resulting pHof 7.9,.and the strength was further increased by the incorporation of2% potassium sulphate in the composition, with a resultant pH of 7.65,and a slightly better plaster set was obtained. Where the borax contentis raised to in the neighborhood of 0.75%, at least 3% K2804 is requiredin order to secure good plaster behavior. In general, a ratio of about1:4 between the borate and alkali-metal sulphate contents has been founduseful.

As far as is known, any chemical compound containing a borate radical isoperative to-secure the desired toughening action, although by virtue ofother properties, such as physiological properties which might preventthe use of the gel composition for oral impressions, some boratecompounds would be objectionable. Among the borate-containing compoundswhich I have employed with complete success, may be listed: bariumborate, borax (sodium tetraborate decahydrate), boric acid, cadmiumborate, calcium borate, chromic borate, ferric borate, ferrous borate,magnesium borate, manganese borate, and zinc borate.

Compositions containing a suitable borate compound and having a pH.value within the desired range may be secured by any one of severalmeans. For example, I have found that an alkaline borate suchas boraxmay be used directly, or may be partially neutralized with an acid, oracid salt, or reacted with some other metallic compound or salt whichwill produce a resultant borate compound which will impart the desiredpH to the gel. Sulphuric acid serves as an'example of a satisfactoryacid, and sodium or potassium acid sulphate serve as examples ofsatisfactory acid salts. Conversely, an acid borate compound, such asboric acid, may be buf- 'fered with an alkali, such as sodium hydroxide,to secure the desired pH .value. Examples of metallic salts which may bereacted with an alkaline borate to produce a modified borate of thedesired properties willbe given in the following description. Theselatter materials are in general the more convenient and practical, inView of the factthat the proportion thereof in the gel material may bevaried within comparatively wide limits without bringing the pH-value ofthe resulting gel outside of the desired range, while the amount of anacid or acid salt which is added to neutralize an alkaline borate asabove described must be very accurately controlled, and the compoundingsteps involved in the addition of the alkaline borate and a bufferingacid constituent (or an acid borate and a buffering alkali) are somewhatmore complicated.

Certain-borates give pH values within the more desired range of pH.7.9'to 8.2 when dissolved or suspended in water; among these may be listedbarium borate, cadmium borate, and manganese borate. It has further beennoted that the method of preparation ofthe borate will afiect itscomposition and will therefore afieet the results obtained with it.Relatively little is known about the chemical constitution or formulaeof the different metallic borates; for example, it is known that theproduct obtained from the reaction between a manganese salt and sodiumtetraborate (in aqueous solution) is not manganous tetraborate as maybeexpected, but is ordinarily monohydrated manganous tetrahydroorthoborate. Barium and cadmium probably form similar borates.

The dental impression composition of the present invention is one whichconsists principally of a reversible hydrocolloid gel, and moreparticularly a gel of a vegetable mucilage, such as agar-agar, Irishmoss, or Iceland moss (all of which are polymers of derivatives ofhexose sugars). The composition also contains a small proportion,generally in the neighborhood of one per cent or less of aborate-containing reagent, which may be either added to the otherconstituents of the composition or formed in the compounding process,and the pH value of the composition is between the approximate limits ofpH 6.8 to pH 8.5. The composition prefererably also contains 1% or more,as from 2% to 4%, of an alkali-metal sulphate, when the pH is outsidethe range of pH 7.9 to 8.2 while still within the broad approximaterange of pH 6.8 to pH 8.5.

It will be appreciated that materials such as glycerine, coloringmaterials, flavoring agents, and/or fillers may also be incorporated inthe composition. Various water-soluble food dyes 40 are available forcoloring the mix, and anything from anise to Wintergreen may be employedas a fiavoring: material. Suitable antiseptic agents may also be addedif desired; in any case the addition of flavoring materials orantiseptic agents should not be so great as to give the final product adisagreeable or strong odor or taste. Such fillers as zinc oxide,titanium oxide, finely dispersed rubber compounds, emulsified resins andthe like, may be employed as fillers. Fibrous ma- 50 terials, such aswood fiber may also be added.

As a specific example of a dental hydrocolloid according to thisinvention, I may make up the following mixture:

Pounds Agar-agar (No. 1 Kobe, strip) 30.00 Borax (technical) 0.42Potassium sulphate 4.20

Water, q. s. to make 210.00

The agar-agar is washed in fresh water for about half an hour and thendrained. The wet agar-agar is heated and stirred in a steamjacketedkettle until a smooth sol is obtained. The borax (as a saturatedsolution in water) is then added slowly, with constant stirring, and thepotassium sulphate is then added. The stirring and heating is continueduntil the product is smooth and homogeneous, and water is added orevaporated until a final weight of 210 lbs. results; The material maythenbe packaged for shipment. The resulting pH value of the abovemixture will be in the neighborhood of pH 8.3, depending somewhat uponthe character of.the agar-agar, the pH of this raw material having beenfound to vary to. some degree, and the pH 7 of the water used.

As a further example of a composition prepared according to the presentinvention, using a metallic borate, I may prepare manganous borate byadding a solution of manganese chloride to a solution of borax,precipitating the manganous borate, which is separated by decantation,filtering ando washing, and drying if desired, before addition to theagar mass. The composition employed may be as follows:

Pounds Agar-agar (No.1 Kobe, strip) 30.00 Manganous borate, powder 1.05Potassium sulphate 4.20 Water, q. s. to make 210.00

The agar-agar is washed in fresh water for about half an hour and thendrained. In this washing the agar-agar soaks up almost all the waterneeded for the batch, so that ordinarily no additional water isnecessary. The wet agar-agar is heated in a steam-jacketed kettle andstirred constantly at a moderate rate. After an hour or two of heatingand stirring, the agaragar will have forme a fairly smooth sol. Hotwater may be added at this point, if necessary, to bring the totalweight of the material to a little over 210 pounds. The manganous boratepowder is added at this time, as by sitting through a fine mesh screen(or by ball-milling in a small quantity of water, and adding the wetmix), and added to the agar-agar sol under continuous stirring. Thepotassium sulphate may then be added in the form of fine crystals orpowder or in water solution. The stirring and heating is continued untilthe manganous borate is uniformly distribtued and .the potassiumsulphate is dissolved (if added as a solid), and longer if necessary toevaporate any excess water which may have been added, so as to end witha final weight of 210 pounds. The hot sol may then be filled intosuitable containers and packaged for shipment.

Commercial manganous borate powder may be employed, if desired, in whichcase it is desirable that a very good grade be selected inasmuch asimpurities present in the material may have considerable efiect on theproperties of the final gel.

As another example of the preparation of a dental impression compositionaccording to the present invention I may employ a mixture of:

Pounds mar-agar (No. 1, Kobe, strip) 30.00 Borax .70 Barium chloride(BaClz.2I-IzO) .93 Potassium sulphate 4.20

Water, q. s. to make 210.00

The agar-agar gel may be made up as described in connection with thefirst example, and the barium chloride added in crystal form and themixture stirred until the barium chloride is dissolved, after which thedry borax is added a little at a time. This method requires a strongstirring apparatus to keep the material properly stirred, as the boraxwill cause the material to become extremely viscous if added toorapidly. A slow and careful addition of the borax is desirable. bariumborate may be prepared by separately reacting the barium chloride withthe borax, filtering and washing the material, and adding the whiteprecipitate to the gel as in accordance with the first describedexample.

As a further example of the preparation of a material according to thepresent invention, I may take 30 lbs. of agar-agar prepared as above Asa modification of this method, the

described, .7 lb. of boric acid, 4.2 lbs. of potassium sulphate, andsufiicient sodium hydroxide to establish the pH value of the material atthe correct range. According to this procedure, the agar-agar gel isworked up hot as described in connection with the first example, and theboric acid is dissolved in the hot sol and thoroughly mixed. The sodiumhydroxide is then added to the mixture, preferably as a 5% solution orthe like, and small samples are withdrawn from the batch and allowed togel, and pH determinations made thereon, adding the sodium hydroxideuntil the pH value of the material is brought to within the desiredrange. The potassium sulphate may then be dissolved in the sol and theheating and stirring continued until the final weight is 210 pounds,water being evaporated or added as necessary.

As another example of the practice of the present invention, acomposition may be made up with agar-agar, borax, sulphuric acid,potassium sulphate, and water, in a manner comparable to thatimmediately above described, in which the borax is buffered with thesulphuric acid until the desired pH value is secured. In the practice ofthis particular procedure, it is desirable to add a small portion of theborax to the sol while stirring, then a little sulphuric acid, until thesol is no longer sufiiciently alkaline to be colored pink withphenolphthalein, after which more of the borax is added and moresulphuric acid, while checking with the indicator, until all of theborax has been added. The final neutralization or bufiering with thesulphuric acid is made very carefully and checked by cooling some of thesol as in the above example and checking the pH value of the cold gel.The potassium sulphate may be added last. If all of the borax were addedin one amount, the sol would become so viscous that it would besubstantially impossible to properly stir the mixture, wherefore thestepby-step alternate addition of borax and sulphuric acid is moreadvantageous. The sulphuric acid, serving as a buffering agent for theborax, is preferably added as a ten per cent solution or there abouts,and a solution of sodium bisulphate or potassium bisulphate mayequivalently be substituted in this connection. Ifthis substitution ismade, the proportion of alkali metal sulphate added to secure thedesired plaster-hardening action may be correspondingly reduced.

As a further example, a composition of agaragar, water and manganousborate powder may be prepared, in the absence of addedplaster-accelerating agent such as the above-described alkali metalsulphate. For example, I have prepared a suitable composition bycompounding 0.21 lb; of manganous borate powder in an agar-agar mixturecontaining 30 lbs. of agar-agar, and prepared according to the firstexample above.

The p H values given herein are the values exhibited by the gel materialin solidified state, i. e., at room temperature (20 C.) The pH values atelevated temperatures (with the material in sol condition) are somewhatlower, and in addition to the diifioulty of accurate pH determinationson the material in sol condition, the pH values at such elevatedtemperatures do not appear to consistently constitute an indication ofthe desired properties of the material. The data given herein are basedon electrometric pH determinations, using a gold electrode, bytriturating grams of gel with 10 cc. of pH 7 .0 distilled water,allowing to stand at room temperature for two hours, adding a smallamount of quinhydrone, and taking readings at approximately C. It ispossible that other values of pH may be obtained by modified testprocedures, and for this reason the described and claimed pH range inthe materials will be considered as having been determined by the aboveprocedure.

It will further be appreciated that the compounding order set forth inthe above examples is not critical, and may be modified in accordancewith the preference of and the facilities available to the compounder.As a specific example of such modification, I have dissolved all thesoluble reagents to be employed in a given composition in the waterwhich is used to swell the dry agaragar, wherefore when the compositionis heated to melt the agar-agar, a completely homogeneous gel is morereadily secured than where dry reagents or concentrated solutionsthereof are added.

I claim:

1. A dental impression composition consisting principally of areversible hydrocolloid gel and containing a borate-containing reagentin an amount not exceeding about 1% and the pH value of such compositionbeing within the range of about pH 6.8 to pH 8.5.

2. A dental impression composition consisting I principally of areversible hydrocolloid gel, and containing a plaster-hardening agentand a borate-containing reagent, said reagent being present in saidcomposition in an amount not exceeding about 1% and the pH value of suchcomposition being within the range of about pH 6.8 to pH 8.5.

3. A dental impression composition of the hydrocolloid type having highstrength characteristics in the gel condition, which comprises: areversible hydrocolloid vegetable mucilage gel modified by aborate-containing reagent in solution, and having a pH value within therange of about pH 6.8 to pH 8.5, said composition being one which willsecure an effective plaster set against a dental grade of gypsum plasterwithin about minutes.

4. A dental impression composition as set forth in claim 3, andcontaining in addition a plaster hardening agent.

5. A dental impression composition as set forth in claim 3, andcomprising in addition 1% or more of an alkali metal sulphate insolution.

6. A dental impression composition consisting principally of areversible hydrocolloid gel and containing a borate-containing reagentin an amount not exceeding about 1%, and having a pH value within therange of about pH 7.9 to pH 8.2.

7. A dental impression composition as set forth in claim 6, in whichsaid borate-containing reagent comprises a borate compound added in suchproportion as to establish the pH of said gel at a value within saidrange.

8. A dental impression composition as set forth in claim 1 andcontaining in addition a plaster hardening agent, the ratio ofborate-containing reagent to plaster-hardening agent being in theneighborhood of not less than 1:4.

9. A dental impression composition consisting principally of areversible hydrocolloid gel of a vegetable mucilage and containingmanganous borate in an amount not exceeding 1%.

BENNETT PREBLE.

